1. Field of the Invention
The present invention relates to a paper machine, and, more particularly, to a dewatering tissue press fabric used in a belt press in a paper machine. The present invention also relates to a dewatering tissue press fabric for use in a high tension extended nip around a rotating roll or a stationary shoe and/or which is used in a papermaking device/process. The present invention also relates to a press fabric for the manufacture of tissue or towel grades utilizing a through-air drying (TAD) system that is engineered to provide a very flat, even surface yet with a high level of resilience and resistance to compaction. The fabric has key parameters which include permeability, weight and caliper,
2. Discussion of Background Information
The manufacture of tissue utilizes an improved technology called TAD, i.e., through air drying process. This process increases paper quality due to the higher bulk of the tissue paper. As a result, TAD sets the standard for high grade tissue.
In a wet pressing operation, a fibrous web sheet is compressed at a press nip to the point where hydraulic pressure drives water out of the fibrous web. It has been recognized that conventional wet pressing methods are inefficient in that only a small portion of a roll's circumference is used to process the paper web. To overcome this limitation, some attempts have been made to adapt a solid impermeable belt to an extended nip for pressing the paper web and dewater the paper web. A problem with such an approach is that the impermeable belt prevents the flow of a drying fluid, such as air through the paper web. Extended nip press (ENP) belts are used throughout the paper industry as a way of increasing the actual pressing dwell time in a press nip. A shoe press is the apparatus that provides the ability of the ENP belt to have pressure applied therethrough, by having a stationary shoe that is configured to the curvature of the hard surface being pressed, for example, a solid press roll. In this way, the nip can be extended 120 mm for tissue, and up to 250 mm for flap papers beyond the limit of the contact between the press rolls themselves. An ENP belt serves as a roll cover on the shoe press. This flexible belt is lubricated by an oil shower on the inside to prevent frictional damage. The belt and shoe press are non-permeable members, and dewatering of the fibrous web is accomplished almost exclusively by the mechanical pressing thereof.
WO 03/062528 (whose disclosure is hereby expressly incorporated by reference in its entirety), for example, discloses a method of making a three dimensional surface structured web wherein the web exhibits improved caliper and absorbency. This document discusses the need to improve dewatering with a specially designed advanced dewatering system. The system uses a Belt Press which applies a load to the back side of the structured fabric during dewatering. The belt and the structured fabric are permeable. The belt can be a spiral link fabric and can be a permeable ENP belt in order to promote vacuum and pressing dewatering simultaneously. The nip can be extended well beyond the shoe press apparatus. However, such a system with the ENP belt has disadvantages, such as a limited open area.
It is also known in the prior art to utilize a through air drying process (TAD) for drying webs, especially tissue webs. Huge TAD-cylinders are necessary, however, and as well as a complex air supply and heating system. This system also requires a high operating expense to reach the necessary dryness of the web before it is transferred to a Yankee Cylinder, which drying cylinder dries the web to its end dryness of approximately 97%. On the Yankee surface, also the creping takes place through a creping doctor.
The machinery of the TAD system is very expensive and costs roughly double that of a conventional tissue machine. Also, the operational costs are high, because with the TAD process it is necessary to dry the web to a higher dryness level than it would be appropriate with the through air system in respect of the drying efficiency. The reason is the poor CD moisture profile produced by the TAD system at low dryness level. The moisture CD profile is only acceptable at high dryness levels up to 60%. At over 30%, the impingement drying by the hood of the Yankee is much more efficient.
The max web quality of a conventional tissue manufacturing process are as follows: the bulk of the produced tissue web is less than 9 cm3/g. The water holding capacity (measured by the basket method) of the produced tissue web is less than 9 (g H2O/g fiber).
The advantage of the TAD system, however, results in a very high web quality especially with regard to high bulk, water holding capacity.
What is needed in the art is a belt, which provides enhanced dewatering of a continuous web.
WO 2005/075732, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a belt press utilizing a permeable belt in a paper machine which manufactures tissue or toweling. According to this document, the web is dried in a more efficient manner than has been the case in prior art machines such as TAD machines. The formed web is passed through similarly open fabrics and hot air is blown from one side of the sheet through the web to the other side of the sheet. A dewatering fabric is also utilized.
WO2005/075736 discloses an ATMOS system which uses a belt press. A dewatering fabric is disclosed as an important feature of the system.
The use of a press fabric is well known in standard tissue-making systems. In such systems, the fabric acts to dewater the sheet by acting as a way to move the water from the sheet to one or more dewatering devices. Known systems include a press formed by a smooth non-perforated roll and a grooved or drilled counter roll.